Various zirconium alloys are used as structural components in the nuclear industry. The most commonly used alloys, Zircaloy-2 and Zircaloy-4, contain strong alpha stabilizers tin and oxygen, plus the beta stabilizers iron, chromium and nickel. These alloys are generally forged in the beta region, then solution treated at about 1065.degree. C. (1950.degree. F.) and water quenched. Subsequent hot working and heat treating is done in the alpha region (below 790.degree. C.) to preserve a fine, uniform distribution of intermetallic compounds which results from solution treating and quenching. Corrosion resistance in steam and hot water depends on the distribution of the intermetallic compounds.
Another significant commercial zirconium alloy is Zr-2.5Nb. The mechanical and physical properties of Zr-2.5Nb are similar to those of the Zircaloys but the corrosion resistance is slightly inferior to that of the Zircaloys.
In zirconium, the low-temperature alpha phase has a close-packed hexagonal crystal structure which transforms to a body-centered-cubic structure at about 870.degree. C. (1600.degree. F.). The transformation temperature is affected by even small amounts of impurities such as oxygen. Alpha-stabilizing elements raise the temperature of the allotropic alpha-to-beta transformation. The alpha-stabilizing elements include Al, Sb, Sn, Be, Pb, Hf, N, O and Cd. Beta-stabilizing elements lower the alpha-to-beta transformation temperature. Typical beta-stabilizers include Fe, Cr, Ni, Mo, Cu, Nb, Ta, V, Th, U, W, Ti, Mn, Co and Ag. Low-solubility intermetallic compound formers such as C, Si and P readily form stable intermetallic compounds and are relatively insensitive to heat treatment.
In addition to being an alpha-stabilizing element, oxygen is also used for solid-solution strengthening of zirconium. The oxygen content of Kroll process sponge generally varies from about 500 to 2000 ppm depending on the number of purification steps and the effectiveness of each step. Crystal bar zirconium generally contains less than 100 ppm oxygen. For instance, Table 5.10 of The Metallurgy Of Zirconium, by B. Lustman et al., McGraw-Hill Book Co., Inc., 1955, sets forth a typical analysis of Westinghouse crystal-bar zirconium having 200 ppm oxygen, 200 ppm Fe, 30 ppm Si, 30 ppm Al, 40 ppm Hf, less that 0.5 ppm Cu, 10 ppm Ti, less than 50 ppm Ca, less than 10 ppm Mn, less than 10 ppm Mg, less than 10 ppm Pb, less than 10 ppm Mo, 30 ppm Ni, 30 ppm Cr, less than 10 ppm Sn, 10 ppm N, 20 ppm H and 100 ppm C and elements not detected included Ga, Co, W, Au, Ag, Ta, Cb, B, V, P, Bi, Cd, Y, Yb, In, Ir, As, Os, Lu and Na.
Gallium is used predominantly in the electronics industry where it is combined with elements of Group III, IV or V of the periodic table to form semiconducting materials. Gallium in aluminum causes severe intergranular corrosion of the aluminum.
Zirconium alloys are disclosed in U.S. Pat. Nos. 3,148,055; 4,584,030; 4,707,330; 4,717,434; 4,751,045; 4,778,648; 4,810,461; 4,863,679; 4,908,071; 4,938,920; 4,938,921; and 4,963,316. U.S. Pat. No. 4,659,545 discloses a zirconium-based nuclear fuel rod cladding. U.S. Pat. No. 3,777,346 discloses a tension band for suspending rotatable mechanisms of measuring instruments, the tension bands being composed of Ti, Zr and Hf alloys which may also contain up to 15 atomic percent of non-transition metals such as Al, Sn, In, Ga or Cu.